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A medium-power helicon thruster (MPHT) is an electric propulsion device that uses a helicon plasma source to achieve specific impulses of up to 1500 s with argon propellant. 1 The Center of Studies and Activities for Space (CISAS) has created a prototype design, and the Georgia Institute of Technology (GA Tech) has designed and built a nominally 1.5-kW MPHT that sustains high density, steady-state plasma over a range of operating conditions. All tests are performed in the GA Tech vacuum test facility at an operating pressure less than 3.1 x 10-5 Torr-Ar. Ion number density, electron temperature, and electron energy distribution function measurements are taken with an RF-compensated Langmuir probe as a function of RF frequency (2-15 MHz), RF forward power (0-1.5 kW), magnetic field strength (0-1100 Gauss), and argon mass flow rate (0.74-4.45 mg/s). The maximum ion number density is 8.0 x 10 18 m-3. The measurements are compared with several known helicon plasma sources. Experimental characterization of this device allows validation of the 1-D and 2-D codes developed at CISAS, which simulate plasma acceleration during vacuum expansion.

A medium-power (1.5 kW) plasma thruster based on a helicon source is considered as a candidate for primary space propulsion. A high-density plasma is produced by the use of a radio frequency (RF) transmitting antenna, which produces helicon waves to ionize a neutral gas (e.g., argon, krypton, xenon, helium or hydrogen) flowing through a tube and confined by a magnetic field. The plasma is accelerated through a potential drop created by a divergent magnetic field, giving a sudden reduction in electron density (and hence plasma potential) very close to the open end of the source tube. The plasma may then expand through a "magnetic nozzle" into the vacuum. Numerical studies are conducted by CISAS in order to investigate the physics connected with the potential drop. The analysis is conducted through a combination of 1-D and 2-D numerical codes. The PPDL code is developed and used for the 1-D analysis. The main features of the code are: hybrid Boltzmann electron/drift-kinetic ion, inclusion of dominant 2-D effects, and high computational efficiency thorough implicit non linear Boltzmann solver. The 2-D analysis is performed with XOOPIC, an open source code available from Berkeley University. The combined approach is very useful since the 1-D code is used to screen many different experimental conditions and to identify the correct boundary conditions. The 2-D code is then used to refine 1-D results. The two models, combined with a global model, specifically developed to simulate the plasma reaction inside the plasma source, are run through genetic algorithms to identify an optimal thruster configuration in the 1500-W power regime. In addition, the thruster is thermally and mechanically sized. Nomenclature A = cross surface of the cell revolution A EXH = geometrical exhaust area BSCCO = Bismuth Strontium Calcium Copper Oxide